At Midsummer—also known as the Summer Solstice—the Holly King slays the Oak King to rule over the winter months. The days become shorter as we say farewell to the sun until the Winter Solstice comes around again on December 21st, when mother nature shuts down for her winter slumber. This almost doesn't make sense when you consider those warm summer days, but there you have it.
Both solstices are symbolic of an eternal cycle of fertility, growth, death and rebirth. Happy winter solstice, and hooray for longer days!